The present invention relates to a process and an apparatus for coating a slurry coating material onto a continuous base sheet at a given thickness, and particularly to a process and an apparatus for intermittently coating a slurry coating material to alternately form coated and uncoated areas, of predetermined lengths on a base sheet.
In mass production of, for example, a spiral electrode type lithium battery, the following intermittent coating technique is employed. A band-shaped hoop material of a copper or an aluminum foil serving as a current collector of a battery is used as a base sheet, and a slurry mixture containing an electrode active substance as a main component is used as a coating material. The coating material is applied to the base sheet, for example, over an area having a length of 30 cm at a given thickness in such a manner that the coating material is not applied at all to a subsequent area having a length of 5 cm, thereby alternately forming a coated area having a predetermined length and an uncoated area having a predetermined length on the continuous base sheet.
The most typical process used to carry out such an intermittent coating operation employs a coating device referred to as a reverse roll coater as shown in FIG. 7.
Referring to FIG. 7, a coating roll 1 (hereinafter referred to as a C-roll) is rotated at a predetermined speed in a direction indicated by an arrow a. A backing roll 2 (hereinafter referred to as a B-roll) is in contact with the C-roll 1 with a base sheet 3 interposed therebetween and is rotated in a direction opposite from the direction of rotation of the C-roll 1 (i.e., in a direction indicated by an arrow h). The base sheet 3 is allowed to travel in a direction indicated by an arrow c along the B-roll 2 and to pass between the C-roll 1 and the B-roll 2.
An adjuster referred to as a doctor roll 4 (hereinafter referred to as a D-roll) is fixedly (i.e., non-rotatably) mounted above the C-roll 1 and is spaced a predetermined distance therefrom. A coating material 6 accumulated in a hopper 5 is deposited onto a peripheral surface of the C-roll 1 to pass through a region of a minimum distance between the C-roll 1 and the D-roll 4, whereby the amount of coating material 6 applied to the base sheet 3 is controlled to a value (thickness) corresponding to such distance.
A coating layer 6a controlled to the predetermined thickness by the distance between the C-roll 1 and the D-roll 4 is transferred onto the base sheet 3 when the base sheet 3 is passed between the B-roll 2 and the C-roll 1. In this manner, a coating layer 6b having a controlled constant thickness is continuously applied onto the base sheet 3.
In order to intermittently form uncoated areas, there is mounted an operating mechanism for moving the B-roll 2 in parallel in a direction perpendicular to a rotational axis thereof. Thus, the B-roll 2 is displaced in a direction indicated by an arrow d at every given time interval, until it is sufficiently spaced apart from the C-roll 1 so that the coating layer 6a is not transferred from the C-roll 1 to the base sheet 3. After a lapse of a predetermined time, the B-roll 2 is displaced in a direction indicated by an arrow e back to its original position, and the coating layer 6a on the C-roll 1 again is transferred to the base sheet 3. By repeating such operations, the above-described intermittent coating can be achieved.
However, the above prior art intermittent coating process may suffer from a problem that it is impossible to control the thickness of the coating layer 6b in the coated area to a constant value with high accuracy. This is due to the movement of the B-roll 2 during switching from a coating position to an uncoating position. The amount of coating material 6 applied to the base sheet 3 is controlled basically by the distance between the C-roll 1 and the D-roll 4 and the rotational speeds of the C-roll 1 and the B-roll 2. However, the coating material 6 is applied to the base sheet 3 in a manner that the coating layer 6b is raised along with the movement of the B-roll 2 away from the C-roll 1, immediately before the coating layer 6b applied on the base sheet 3 is discontinued in the transient state of the movement of the B-roll 2 away from the C-roll 1 (in the direction of the arrow d). Consequently, the thickness of a trailing edge of the coating layer 6b is locally increased at the end of the coated area, thereby bringing about a very undesirable result. For example, in a mass production line for the above-mentioned spiral electrode type lithium batteries, local drop-off of the coating mixture is liable to occur at the thickened edge of the coating layer 6b at a step of rolling an intermittently coated sheet-shaped electrode, and a current collector of a thin metal foil in such edge portion is excessively rolled, so that it is liable to be broken. In addition, at a step of spirally winding the sheet-shaped electrode along with a separator, a weaving is liable to occur due to the thickened edge. These phenomena may cause hindrances to smooth production. Furthermore, even after completion of assembling of components into a battery, an optimal electric capacity balance between positive and negative electrodes is lost at the thickened edge portions, and particularly, in spiral type secondary lithium batteries the effective utilization of an active material is disturbed. For example, with such secondary batteries, when thickened edge portions are provided only on the side of the positive electrode, the negative side electric capacity may become smaller than that on the positive side and hence, dendrites are liable to be produced in the thickened edge portions on the side of the negative electrode during charging of those batteries, resulting in a problem of short-circuiting between the electrodes.
The present invention has been accomplished under such a technical background, and it is an object of the present invention to provide an intermittent coating process and an apparatus for use therein, wherein a local increase of the amount of coating material applied can be prevented locally at the end of a coated area, and the thickness can be controlled to be constant with high accuracy over the entire coated area.